Lab Report on the NMR Spectroscopy of Organic Molecule

The lab report below was submitted as part of the coursework for
CM2102 Spectroscopic Applications. Please do not plagiarise from it as
plagiarism might land you into trouble with your university. Do note
that my report is well-circulated online and many of my juniors have
received soft copies of it. Hence, please exercise prudence while
referring to it and, if necessary, cite this webpage.

Aims

To obtain an NMR
spectrum of a liquid mixture consisting of ethyl acetate, p-tolualdehyde and
cyclohexene in CDCl3 solvent, assign each peak of the spectrum to
the different molecules, and to determine the concentration ratio of the
organic compounds in the mixture.

4.Ensure
that air is blowing out, load the NMR tube into the spectrometer

5.Click “Lift-On/Off”
button to turn off gas flow

6.Type “dir”

7.Select
“Standard 1H”

8.Type “edc”

9.Enter the
file name as CM2102 date initials

10.Type
“lock”

11.Select
CDCl3 as solvent

12.Wait for
“lock ready” and start shimming

13.Select Z1
– left or right. Click until the signal is at the highest level.

14.Select Z2
– left or right. Click until the signal is at the highest level.

15.Repeat
steps 14 and 15 until the signal is at the highest level, then press STDBY

16.Type “rga”

17.When “rga”
is ready, type “zgefp”

18.While
waiting, type “setti” to set title for spectrum

19.Type the
title as CM2102 sample 7

20.Wait until
the spectrum is out

21.Type “apk”
to do phase correction

22.Zoom into
the region of the CDCl3 (̴ 7 ppm)

23.Calibrate the CDCl3
peak at 7.26 ppm

24.Click “Lift-On/Off” button
to lift up the sample

25.Remove the sample from
the spectrometer

Results and
calculations

(The effects of the chemical environments on the
chemical shifts will be explored under the section, “Discussion”.)(For singlet,
doublet and triplet coupling patterns, the chemical shift values given in this
report will be their midvalues. For multiplets, the chemical shift values will
be given as a range.)

For a molecule of ethyl acetate

Proton

Expected
chemical shift, ppm

Observed
chemical shift, ppm

Expected
Coupling

Pattern

Observed
Coupling

Pattern

Integral

Ha

0.7 -
1.3

1.2527

Triplet

Triplet

3

Hb

3.5 -4.8

4.0938
to 4.1364

Quartet

Quartet

2

HC

2 - 2.2

2.0382

Singlet

Singlet

3

Table 1: analysis of ethyl acetate

For a molecule of p-tolualdehyde

Proton

Expected
chemical shift, ppm

Observed
chemical shift, ppm

Expected
Coupling Pattern

Observed
Coupling

Pattern

Integral

Ha

9.0 -
10.0

9.9595

Singlet

Singlet

1

Hb

6.5 –
8.0

7.7732

Doublet

Doublet

2

HC

6.5 -
8.0

7.3283

Doublet

Doublet

2

Hd

2.3 –
2.7

2.4376

Singlet

Singlet

3

Table 2: analysis of p-tolualdehyde

For a molecule of cyclohexene

Proton

Expected
chemical shift, ppm

Observed
chemical shift, ppm

Expected
Coupling

Pattern

Observed
Coupling

Pattern

Integral

Ha

1.2 –
1.4

1.5944
to 1.6191

Triplet

Multiplet

4

Hb

1.6 -
2.6

1.9800
to 1.9869

Doublet
of triplet

Multiplet

4

HC

4.5 –
6.5

5.6652

Triplet

Singlet

2

Table 3: analysis of cyclohexenes

To determine the relative concentration of
ethyl acetate, p-tolualdehyde and cyclohexene, distinct peaks of each molecule
were identified. Distinct peaks are individual peaks of ethyl acetate,
p-tolualdehyde and cyclohexene that do not overlap at the same chemical
shifts.

The distinct peaks are summarised below.

The peak at = 4.0938 to 4.1364 ppm belongs to
ethyl acetate (Hb).
The peak at = 5.6652 ppm belongs to cyclohexene (Hc).
The peak at = 9.9595 ppm belongs to p-tolualdehyde (Ha).

To
further confirm the relative concentration ratio, the calculations were
repeated for the other peaks of the compounds.

ethyl acetate

p-tolualdehyde

cyclohexene

Ha

Hc

Hb

Hc

Hd

Ha

Hb

No. of H per molecule

3

3

2

2

3

4

4

Approximate integral
ratio at corresponding chemical shift

3

3

2

2

3

4

4

Relative
concentration of molecule present

1

1

1

1

1

1

1

Discussion

Nuclear
Magnetic Resonance (NMR) spectroscopy makes use of the magnetic properties of
certain nuclei such as proton to obtain information regarding the nature of the
immediate environment of the magnetically distinct atom. NMR spectroscopy is
the most important and widely used technique by chemists for identifying the
structures of organic and inorganic compounds because it allows a full analysis
of the entire spectrum for a compound.

In
this experiment, the 1H NMR spectrum was obtained to determine the
concentration ratio of the three compounds in the mixture. Depending on its chemical environment, the protons in a molecule
resonate at slightly different frequencies. The variation of the resonance
frequency with chemical environment is known as the chemical shift. All protons
in chemically identical environments within a molecule are chemically
equivalent and exhibit the same chemical shift. The number of peaks corresponds
to the number of chemically distinct types of protons in the molecule. The area
under each peak is proportional to the number of hydrogens generating that
peak. The protons in different magnetic environments of the same molecule can
interact with one another through spin-spin coupling. This leads to the splitting
of signals. The splitting pattern of a nucleus can be predicted by the (N+1)
rule which states that a nucleus is split by N equivalent nuclei into (N+1)
peaks. The ratio of line intensities within an NMR signal is determined by the
Pascal’s triangle.

Solvent used

Deuterated
solvent CDCl3 was used to avoid the resonance from protons in the
solvent from swamping the proton spectrum of compound. CDCl3 was
chosen because most compounds
dissolve in it, it is also volatile so can be removed easily and it is inert
thus no exchange of its deuterium with protons in the molecule being studied is
possible. In addition, CDCl3 can be used as a reference using the
weak peak at 7.26ppm to ensure that the frequencies generated throughout the
experiment remain constant. A small peak was observed for CDCl3 in
the proton NMR spectrum although deuterium is NMR inactive because there is a
small percentage of CHCl3 that is not deuterated.

Ethyl acetate

Ethyl acetate has an ester functional
group. There are 3 different proton environments for ethyl acetate. Both Hb
and Hc are more deshielded than Ha because they are
closer to the electronegative oxygen atoms. The electron density on hydrogen is
reduced by the electron withdrawing effect of oxygenleads to a reduction in local
diamagnetic shielding that translates into an increase in deshielding and hence
an increase in the chemical shift. The quartet signal, 4.0938 to 4.1364 ppm, is
due to Hb coupling with 3 neighbouring Ha and it is the
distinct peak for ethyl acetate as there are no other peaks in the region to
overlap at the same chemical shift. The methyl group from Ha couples
with 2 Hb to give a triplet at 1.2527 ppm. Hc is
deshielded by the anisotropy field of the carbonyl groups to give a singlet at
2.0382 ppm as there are no neighbouring protons within 3 bonds.

p-tolualdehyde

p-tolualdehyde has a benzene ring and a
carbonyl group with 4 proton environments. Hydrogens attached to benzene ring Hb
and Hc are deshielded by the significant anisotropic field generated
by the circulation of the π electrons in the ring.Both
Hbatoms are chemically and magnetically
equivalent as they are related by symmetry operation and the coupling with
other protons is the same. Hb couples with Hc to form a
doublet at 7.7732 ppm. As Hc is more distant from the
electron-withdrawing carbonyl group than Hb, Hc experiences a smaller chemical
shift at 7.3283 ppm. Hd is a benzylic hydrogen deshielded by the
anisotropic field of the ring but they are further away from the ring, thus the
effect is smaller. A singlet is
observed at 2.4376ppm as there is no neighbouring ptotons within 3 bonds to
couple with.

The chemical shift of
the proton in the aldehyde group, Ha is at 9.9595 ppm. The high
chemical shift is due to the anisotropy effect of the carbonyl group, the
inductive effect of the electronegative oxygen decreases the electron density
on the attached proton. Furthermore, Ha is
bondedwith a sp2 hybridized carbon thus it has more s-character than
a sp3 hybridized carbon. sp2 carbon atom holds electrons
closer to the nucleus than sp3 carbon, this results in less
shielding for the H nucleus. Thus the anisotropy effect and hybridization affect deshielded Ha therefore its chemical shift is towards a downfield
region. A singlet is observed on this peak because no coupling occurred between
its nearest neighbouring proton (Hb) as they are 4 bonds apart. This
peak is indicative of an aldehyde group since no other protons appear in this
region. An integral value of 1
is expected since there is only one of such proton in the compound.

Cyclohexene

Cyclohexene has an alkene functional group
and a plane of symmetry that is perpendicular to the double bond, thus there
are 3 different proton environments. An alkene has an area of low electron
density in the plane of the molecule because the π orbital has a node there and
the hydrogen nuclei lying in the plane gain no shielding from the π electrons. The
hydrogens attached to the double bond are deshielded due to the anisotropic
field of the π electrons in the double bond. The more distant the hydrogen from
the double bond, the smaller the deshielding effect, therefore the allylic
hydrogen Hb has smaller chemical shifts than Hc. When
cyclohexene adopt the most stable conformation, half-chair,
Hc and Ha can have long range coupling. Therefore, Ha has a higher chemical shift than expected from an ordinary
CH2 group. However, the interaction is almost negligible. Ha is expected to be a triplet as there are 2
Hbfor it to couple with. If a higher resolution NMR spectroscopy is available, Hb is expected to be a doublet of triplets because it can
couple with 2 Ha and a Hc. But because of overlap with
the strong peak from Hc of ethylacetate which has similar chemical
shift, the pattern could not be distinguished easily. The signal appears as a
multiplet. This problem can also be solved by recording the NMR spectrum again
but at a different magnetic field. The distinct peak for cyclohexene is a
singlet at 5.6652 ppm due to Hc. As there are 2 Hc in
each molecule of cyclohexene, the expected integral is 2. For Hc, a
triplet was expected due to coupling with 2 Hb, but instead a
singlet was observed. This could be because when cyclohexene has a half chair
or twisted boatconformation, Hbmay be oriented in away from Hcsuch
a way that there is minimal spin spin coupling.

Determine
the concentration ratio

The NMR spectrum of a mixture of ethyl
acetate, p-tolualdehyde and cyclohexene is a result
of the superimposed spectrum of their individual spectrums. The distinct peaks
selected are 9.9595 ppm, 5.6652 ppm and 4.0938– 4.1364ppm which belong to
p-tolualdehyde, cyclohexene and ethyl acetate respectively. Using the experimentally
determined integration ratio of the distinct peaks, the relative concentration
of p-tolualdehyde, cyclohexene and ethyl acetate can be determined. The results
were then double checked with the integration ratios observed from other peaks.

Conclusion

Proton NMR of a liquid mixture consisting of
ethyl acetate, p-tolualdehyde and cyclohexene in CDCl3 solvent was
obtained and the peaks were assigned. The relative concentration ratio of ethyl
acetate p-tolualdehyde and cyclohexene was found to be 1: 1 : 1 based on
integration of the distinct peaks.